Rheostat



United States Patent Olice 3,156,889 RHEGSTAT Keith E. Starner, Gardena,Calif., assigner to The Aerospace Corporation, Los Angeles, Calif., acorporation of California Filed .lune 14, 1962, Ser. No. 202,483 Claims.(Cl. SSS-151) This invention relates to rheostats and more particularlyto high current, low impedance, innitely variable resistance devices.

Prior art rheostats are designed with many characteristics suitable forperforming numerous specialized operations. Although forced cooling otrheostats is known in environments requiring resistance dissipation oflarge amounts of power and rheostats utilizing mercury pool contacts areknown, a combination of these arrangements to provide control ofhundreds or thousands of amperes has not been fully exploited. Thereremains a problem of obtaining infinitely variable low impedance controlof large battery currents of up to several thousand amperes.

Therefore, an object of the present invention is to provide an improvedhigh current, innitely variable, liquid contact rheostat.

In accordance with one embodiment of my invention, a plurality of hollowarcuate resistive elements are rotatable to engage mating mercury pools.The mercury electrically couples the resistive elements to a contactor,thus forming a rheostat. Means are provided for pumping through theelements and the contactor respectively, a cooling liuid which resultsin a capability of this invention for regulating large amounts ofcurrent without destructive heating of the conducting resistiveelements. A simple arrangement for accomplishing necessary parametercontrol is attained by sensing the critical parameter and adjusting thepositions of the rheostat resistive elements to accomplish anycorrection necessary. For instance, the load current may be sensed withthe rheostat position being adjusted to maintain a desired constantcurrent flow despite impedance variations of the system because ofthermal variations of impedance components.

The subject matter which is regarded as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, as to its organization andoperation, together with further objects and advantages thereof, willbest be understood by reference to the following description taken inconnection with the accompanying drawing in which:

FIG. 1 is a simpliied schematic diagram illustrating one use of thepresent invention in a parameter sensitive environment requiringsubstantial current and power regulation;

FlG. 2 is a side elevation view partially in section of one embodimentof my invention; and

FIG. 3 is a cross-section View taken along the line of 3-3 of FIG. 2.

Referring now to the drawing, wherein like numbers refer to similarparts, i show in FIG. 1 a rheostat 10 of the present invention arrangedin a circuit diagram. In this circuit an arc plasma generator 12 isarranged to apply heated gas to a test model 14. Ofter such anarrangement will be utilized to test ablation characteristics, whereinthe average temperature of the gas impinging on the model 14 is of theorder of 3,000 to 5,000 K. The attaining of such a temperature usuallyrequires electron temperatures within the arc of the order of 15,000 to20,000" K.

The arc generator 12 comprises two electrodes 15 and 15. The electrode15 is preferably ol' a refractory metal such as tungsten butin someapplications may be a corn- CTL 3,155,889 Patented Nev. 1o, 1964 pactedgraphite rod, and the electrode 16 is a subsonic or a supersonic nozzlearrangement formed of a conductive material such as certain copperalloys. Other primary circuit components include a battery power supply18 having an internal resistance represented schematically by a rheostat19, a double contact circuit breaker 20 and a fixed ballast resistor 22.The circuit breaker 20 is closed to initiate current flow and is biasedopen upon release by the action of trip solenoid 23 to terminate currentflow. The ballast resistor 22 is provided to limit initial maximumcurrent flow.

The maintaining of a predetermined flow of heated gas from the arcgenerator 12 turns out to be a relatively complex problem. Gas suppliedthrough line 24 is applied to the arc, preferably through a plurality ofchoked supersonic nozzles 25 exiting into an insulating housing 26,whereby the particular internal pressure of the system will notmaterially aliect the rate of gas flow. The gas tiows from the housing26 through the region of the arc and is heated thereby. Several gasesmay be used in an arc enerator of this type including air, argon, heliumand nitrogen. However, the rate of flow of gas 24 may not be varied overa wide range for a given arc without suffering extinguishing of the arc.Therefore, an additional plenum chamber 28 is provided wherebyadditional amounts of gas may be introduced to the system through acontrol valve 29 to regulate the total mass of gas tiow and thetemperature of the gas impinging on the model 14. The plenum chamber 28also operates much in the manner of a muiiler to diiiuse shock gasfronts and dampen the eiects of arc lluctuation and, further, provides aregion of low velocity suitable for measuring the stagnation pressure ofthe gas jet.

Use of the battery power supply 1S provides a desirable source ofripple-free current not readily attained when using alternating currentor rectifier-type power supplies. However, batteries supply current atan essentially constant voltage and desired smooth current variation isaccomplished by a change of the system resistance. For arc generatoroperation requiring a programmed change of current, the ohmic variationis produced by the rheostat 10 of my invention. ln one arrangement, acircuit current signal is generated in a shunt around ammeter 30 andapplied to a reference network 32 which generates an error signal 34.The error signal 34 is applied to a control iield 36 of a selsyn-typepositioning motor 38 which is drivingly coupled to the rheostat 10. Anyerror signal 34 will energize the motor 3S, to reposition the rheostat10 in accordance with a reference voltage applied within the network 32.The reference voltage may be programmed to simulate various phenomena.

In order to produce transient ablation of the test model 14 forsimulation of nose cone re-entry heat transfer rates, programmedvariation of the current supplied to the arc generator 12 will be astandard operating procedure. It is well known in the art that the arcoperating voltage will remain essentially constant over a wide range ofcurrent 'low. Thus the power dissipated in the arc and applied to themodel 14 is directly proportioned to the current controlled by myinvention. By way of example, when the battery power supply 1S iscomprised of 320 12.5 volt storage batteries arranged in a seriesparallel array to give a fixed supply voltage of 250 volts, a current of1500 amperes can flow through the arc generator 12. During suchoperation a voltage of about volts directly across the electrodes 15 and16 is typical.

With the rheostat 10 initially positioned to provide zero ohms theresistance of the rheostat 19 and fixed ballast resistance 22 willdevelop the remaining 150 volt drop of the circuit. By increasing theresistance of rheostat 10 the current can be made to decrease thuschanging the entrasse sa power of the are and the heat-transfer rate tothe model i4. Using specially designed batteries, a typical value oftotal battery internal resistance in such a series parallel array is4.65 (10)*3 ohms leaving 9.53 (10)-2 ohms to 'be made up by the ixedballast resistor 22 to limit the vwill be compensated for automaticallyby operation of the rheostat i in accordance with the error signal 3d.

In the event that excess currents do flow in the circuit,

rthey will also be sensed in the ammeter shunt and applied to anovercurrent relay 39, which will operate the trip solenoid 23 in amanner well known in the circuit breaker art. 0bviously, the errorsignal 34 may be replaced by a manual operation, when the referencenetwork 32 includes an ammeter dial. Also, programming may be manuallyeffected. However, a more sophisticated arrangement as indicated oftenwill provide a smoother transition.

Referring new to FiG. 2, I show in detail the rheostat of my inventionwhich is capable or continuously conducting many hundreds of ampereswhile at the same time having a controlled and infinitely variableimpedance. Empirical data indica-te that a stepped variation of theimpedance in the circuit adversely affects the operation of the arc jetand often produces inaccurate test data. Therefore, I have provided aplurality of rotatable annular nichrome tubular resistive elements dil.Using nichrome elements having a peripheral length of about 20" and adiameter of 1A with a wall 4thickness of .005, the resistive elements 40will each continuously conduct about 400 amperes when they areforced-circulation cooled by room temperature water at 60 psi. and at aiiow rate therethrough of l0 per second. The iiow rate of l0 per secondis approximately 12 gallons per minute when using eight resistiveelements -as shown in FIG. 2. A pump 4l easily supplies this flow ratein a completely pressurized system, and by enlarging the pump al theoutput water may be dumped in arrangements where no large storagecapacity is available. Also, in using the present invention, Wheredistilled water is unavailable scaling should be removed periodically.

In this particular application the resisitive elements 40 each have amam'mtlm resistance of about 0.2 ohm and are arranged each to conduct asmuch as 400 amperes without destructive heating. The coolant water flowsin the direction of arrows 4t2 through a coupling d3 into a rotatablecentral support pipe d4, through insulating coupling pipes 46, andelbows 47, through the nichrome resistive elements 40 and elbows 49 to acoolant header d8 and out through a coupling 50. The coolant header 48also functions as a current collector bus bar `for coupling together aplurality of the resistive elements d0. In one arrangement the couplingpipes d6 are of Teiion and the other coupling elements are chrome-platedcopper tubes and elbows. Chrome plating is resistive to mercury vaporcorrosion. In order that high pressure water may be pumped through thissystem7 a rotatable seal is provided in the form of O-rings 5l and 52 ateach of the couplings 43 and 50 respectively. Also, the pipe 44 isprovided with a plug 53 for directing the water through the resistiveelements 40.

As shown in FIG. 2, the mating contactor element comprises a mercurywell contacter arrangement 54 defining a series of depressions or wells55 which cont-ain pools 56 of mercury to couple electrically theresisitive elements 40 to the contacter 5d. In order that the resistiveelements l0 may be serially coupled, the contactor 5d is divided into aplurality of sections by insulating inserts or washers Thus, the flow otcurrent as illustrated by dashed arrows 59 may be from left to right(FIG. 2) through the illustrated rheostat lil, from a terminal oil tothe irst section of the contacter 54, through the first pair ofresistive elements d0 and elbows d@ to the bus bar d8, through a secondpair of resistive elements 40 to the second section of the contacter 5d,through a third pair of the resisitive elements di), etc. The return ofthe current through the central resistive elements dil to the centralsection of the contactor 5d is assured by the insertion of an insulatingcoupling 62 which divides the bus bar 4S into `a plurality of sections.

Finally, the current iiows from the last section of the contacter 5d tothe output terminal 64. Obviously, the current ilow may be arranged topass between the contactor sections and theV bus bar sections a greaternumber oi times than that illustrated herein. Similarly, additionalresistive elements may be stacked in parallel when it is necessary toprovide increased current capacity.

it is preferable to arrange 4for the water coolant to iiow in thedirection of the arrows d2 (FIG. 2, 3) so that those portions ofresistance elements 42 which are immersed in the mercury pools 56 arefirst to be cooled by the water to thus minimize heat transfer to themercury. In this Yway the creation of noxious and corrosive mercuryvapors can be avoided. For most effective and safest operation the waterinlet temperature should be approximately 10 C. with a flow ratesufficient to keep outlet coolant temperature at or below 80 C. Shouldit be necessary to operate at higher coolant temperatures, in a mannerwell known the coolant system could be pressurized to increase theboiling point of the water and thus eliminate the possibility or"localized boiling in resistance tubes i2- As will be seen in viewingFIGS. 2 and 3 additional cooling for the mercury pools 56 is provided byhaving the area beneath wells SS iacketed. In this jacketed area thepump il also forces the circulation of coolant iiuid in a direction fromleft to right in FIGURE 2.

As discussed in connection with FIG. l, regulation oi the angularposition of the resistive elements d0 is controlled by the motor 3S. Asshown in FIG. 2 the motor 38 is coupled `to the rotatable pipe 44through corrosion resistant reduction gearing 66. As a safety measure,eX- cessive temperatures in the rheostat l@ are prevented and theelements i0 are protected from high exit water temperatures or waterpressure failure by a combined temperature and pressure actuated relay67 which is also coupled to the trip solenoid 23 and will energize itinthe event of high temperature or low pressure. Protective controls ofthis type are well known in the heating industry and need not beexplained in detail here. An additional safety feature is provided inthe form of a Enger or stop 68 positioned on the outer surface of therotatable equipment (the trailing edge of at least one elbow d'7) toengage an abutment 69 (FIG. 3) in the well 55 and thus preventsubstantial loss of contact between the mercury pool 56 and the elements40, when a maximum amount of resistance has been provided.

Since I am utilizing mercury contacts, I prefer to en-V ,shows theannular configuration of one resistive element 40, the central locationof the central pipe 44, the insulating pipe 46 and the relative locationof the bus bar header 48. The electrical current flow is illustrated bythe dashed arrow 59 and the cool-ant water ow is illustrated by thesolid arrow d2. FIG. 3 also illustrates insulating support straps 7dwhich engage the elements 40 by means of loops to maintain them a iixedradial distance from central pipe iid. To maintain the arcuateV re-:greased sistance elements di) fixed longitudinally of central pipe itand to generally rigidize the assembly, aligned hollow spacers 77 areinterposed between adjacent straps 7 4i, and tie rods '76 are securedthrough the spacers 77 and the holes in the straps pill. These tie rods76 and spacers 7'7 will withstand any mechanical vibrations and magneticforces that might tend to move the resistance elements relative to eachother and relative to the mercury pools. Using no more than about 20resistive eleLA ents itl with no more than 400 amperes flow through eachelement, additional cross bracing is usually unnecessary.

As is also illustrated in FIG. 3 each of the resistive elements itl isprovided with a substantial arcuate gap 73 which may be positioned inthe region of mercury pool 56 when the equipment is not in use. In thismanner, corrosion of the nichrome tubing caused by extended contact withmercury may be eliminated.

While I have shown and described a particular embodiment of the presentinvention, further modifications may occur to those skilled in this art.For instance, although I have described in some detail one invironmentrequiring the infinitely variable reheostat of my invention, thisinvention may be used in numerous applications for controlling largebattery currents and the like. I desire it understood, therefore, thatmy invention is not limited to the particular form disclosed and Iintend by the appended claims to cover all such modifications which donot depart from the true spirit and scope of my invention.

I claim:

l. An infmitely variable, high current rheostat for regulating hundredsof amperes in an environment sensitive to relatively small currentvariations, comprising:

a plurality of parallel hollow annular resistive elements adaptable forconducting large electric currents and substantial coolant liuid;

a centrally disposed rotatable support pipe coupled to one end of eachof said elements by means of an insulated hollow coupling, respectively;

a bus bar header both electrically and hydraulically coupled to theopposite ends of said elements;

a contacter defining upwardly opening arcuate wells arranged toencompass an arcuate portion of each of said elements respectively;

a mercury pool within each of said wells for electrically coupling saidcontactor to said elements;

insulating means for dividing said contactor into a plurality ofsections so that the only coupling therebetween is through a pluralityof said elements;

means for detecting the current flow through the rheostat; and

means for rotating uniformly said elements relative to.

said contactor to vary the impedance between said mercury pools and saidheader and thereby regulate the voltage between said contacter sectionswhile maintaining a programmed current flow of the order of hundreds ofamperes through the rheostat.

2. An infinitely variable, high current rheostat for use in a circuitarranged to conduct a programmed current iiow of the order of hundredsof amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable forconducting large electric currents and substantial coolant liuid;

a centrally disposed rotatable support pipe arranged to support one endof each of said elements;

means for electrically insulating said elements from said pipe;

a bus bar header both electrically and hydraulically coupled to theopposite ends of said elements;

a contactor defining upwardly opening arcuate wells arranged toencompass an arcuate portion of each of said elements respectively;

a mercury pool within each of said wells for electrically coupling saidcontactor to said elements;

insulating means for dividing said header into a plurality of sectionsso that the only coupling therebetween is through a plurality of saidelements;

means for forcing coolant iiuid through said elements so that it flowsfrom the region of said pools toward said header;

means for detecting the current flow through the rheostat; and

means for rotating uniformly said elements relative to said contacter tovary the impedance between said mercury pools and said header andthereby vary the Voltage between said header sections while maintaininga controlled current flow through the rheostat.

3. An infinitely variable, high current rheostat for use in a circuitarranged to conduct a programmed current iiow of the order of hundredsof amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable forconducting large electric currents and substantial coolant fluid;

a centrally disposed rotatable support conduit arranged to support oneend of each of said elements;

means for electrically insulating said elements from said conduit;

a bus bar header both electrically and hydraulically coupled to theopposite ends of said elements;

a contacter delining upwardly opening arcuate Wells arranged toencompass an arcuate portion of each of said elements respectively;

a mercury pool within each of said wells for electrically coupling saidcontactor to said elements;

insulating means for dividing said contacter into a plurality ofsections so that the only coupling therebetween is through a pluralityof said elements; and

means for forcing coolant ud through said elements so that it iiows fromthe region of said pools toward said header to thereby minimize thetemperature variation of said pools during prolonged operation of therheostat.

4. An infinitely variable, high current rheostat for use in a circuitarranged to conduct a programmed current iiow of the order of hundredsof amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable forconducting large electric currents and substantial coolant lluid;

a rotatable support conduit arranged to support one end of each of saidelements;

means for electrically insulating said elements from said conduit;

a bus bar header both electrically and hydraulically coupled selectivelyto the opposite ends of said elements;

a first terminal;

a first mercury pool for electrically coupling said first terminal toone of said elements;

a second terminal;

a second mercury pool for electrically coupling said second terminal toanother of said elements;

means for forcing coolant iiuid through said elements so lthat it flowsfrom the region of said pools toward said header to thereby minimize thetemperature Variation of said pools during prolonged operation of therheostat; and

lateral support means between said elements for preventing substantialreaction thereof to magnetic forces therebetween.

5. An infinitely variable, high current rheostat for use in a circuitarranged to conduct a programmed current flow of the order of hundredsof amperes, comprising:

a plurality of parallel hollow annular resistive elements adaptable forconducting large electric currents and substantial coolant fluid;

a rotatable support conduit arranged to support one end of each of saidelements;

means for electrically insulating said elements from `said conduit;

7 a bus bar header both electrically and hydraulically coupled to theopposite ends of said elements; a rst terminal; a rsvt mercury pool forelectrically coupling said irst terminal to one of said elements; asecond terminal;

a second mercury pool for electrically coupling said second terminal toanother of said elements; and means for forcing coolant lluid throughsaid elements so that it ows from the region of said pools toward saidheader to thereby minimize the temperature variation of said poolsduring prolonged operation of the rheostat.

References (liteit in the file of this patent UNITED STATES PATENTS VonBrockdort Y Feb. 26, Smith lluly 20, Emmet May 27, Sykes Feb. 26, KennenSept. 2, Richardson Feb. 24,

FOREIGN PATENTS Germany Feb. 28,

1. AN INFINITELY VARIABLE, HIGH CURRENT RHEOSTAT FOR REGULATING HUNDREDSOF AMPERES IN AN ENVIRONMENT SENSITIVE TO RELATIVELY SMALL CURRENTVARIATIONS, COMPRISING: A PLURALITY OF PARALLEL HOLLOW ANNULAR RESISTIVEELEMENTS ADAPTABLE FOR CONDUCTING LARGE ELECTRIC CURRENTS ANDSUBSTANTIAL COOLANT FLUID; A CENTRALLY DISPOSED ROTATABLE SUPPORT PIPECOUPLED TO ONE END OF EACH OF SAID ELEMENTS BY MEANS OF AN INSULATEDHOLLOW COUPLING, RESPECTIVELY; A BUS BAR HEADER BOTH ELECTRICALLY ANDHYDRAULICALLY COUPLED TO THE OPPOSITE ENDS OF ELEMENTS; A CONTACTORDEFINING UPWARDLY OPENING ARCUATE WELLS ARRANGED TO ENCOMPASS AN ARCUATEPORTION OF EACH OF SAID ELEMENTS RESPECTIVELY; A MERCURY POOL WITHINEACH OF SAID WELLS FOR ELECTRICALLY COUPLING SAID CONTACTOR TO SAIDELEMENTS; INSULATING MEANS FOR DIVIDING SAID CONTACTOR INTO A PLURALITYOF SECTIONS SO THAT THE ONLY COUPLING THEREBETWEEN IS THROUGH APLURALITY OF SAID ELEMENTS; MEANS FOR DETECTING THE CURRENT FLOW THROUGHTHE RHEOSTAT; AND MEANS FOR ROTATING UNIFORMLY SAID ELEMENTS RELATIVE TOSAID CONTACTOR TO VARY THE IMPEDANCE BETWEEN SAID MERCURY POOLS AND SAIDHEADER AND THEREBY REGULATE THE VOLTAGE BETWEEN SAID CONTACTOR SECTIONSWHILE MAINTAINING A PROGRAMMED CURRENT FLOW OF THE ORDER OF HUNDREDS OFAMPERES THROUGH THE RHEOSTAT.